If for some reason you want to sound like them, all you really have to do is use the following words as often as possible:

うち (uchi)

This word is used instead of 私 (watashi), meaning “I/me.” Many more conservative adults consider this improper though, because in standard Japanese うち actually means “we/us,” as in “our family,” “our company,” “our school,” etc.

プリクラ撮りに行くの？うちも行きたい！
Purikura tori ni iku no? Uchi mo ikitai!
Are you going to take purikura? I want to go too!

Purikura are photo booth pictures that can be decorated and turned into stickers. This one is from the popular schoolgirl pop group AKB48.

The construction 〜んだけど (~n da kedo) is often used by schoolgirls to express emphasis and/or indignation at the end of a sentence. Be careful though, because when 〜んだけど comes in the middle of a sentence (before a comma) it simply means “but”!

うざい (uzai)

This word is probably best translated as “annoying,” but its usage is far more varied than any equivalent we have in English.

放課後に出かけたいんだけど、親がうざいからすぐ帰らないと怒られる。
Houkago ni dekaketai n da kedo, oya ga uzai kara sugu kaeranai to okorareru.
I want to go out after school, but my parents are on my case so I have to go straight home or they’ll get mad at me.

あの子ってモデルかな？超きれい！
Ano ko tte moderu kana? Chou kirei!
I wonder if that girl is a model? She’s so pretty!

〜さ (~sa)

This is a conversational particle added to the ends of sentences or phrases. It indicates that the speaker is engaging the other person in the conversation. But some schoolgirls seem to use it in just about every sentence!

There are a few other things that I don’t have the space to cover in this post, but the above words alone should allow you to understand an amazingly large percentage of schoolgirl speak. Just use them with caution, because unless you’re actually a schoolgirl yourself, talking like this will probably just make you sound totally uzai.